This invention relates to an apparatus and method for separating a single sheet from a stack and conveying it by utilizing a suction device which applies an air suction to a stack of sheets to separate a single sheet therefrom for conveyance to a given location.
A sheet separating apparatus utilizing a suction device is well known in the art. In the prior art practice, the separation of a single sheet from a stack is reliably achieved by utilizing a suction device with a photosensitive sheet having a photosensitive material applied to its surface or other special paper sheets treated in a manner which lowers their air permeability. However, such apparatus are not suitable for use in combination with many printing apparatus, for example printing apparatus of the ink jet type, which use sheets of a common paper having no such surface treatment, because the air permeability of such sheets can cause more than one superimposed sheet to be attracted to the suction device or no sheet to be attracted to the latter, failing to satisfy the requirement that a single sheet be fed.
On the other hand, with a conventional sheet separation system, it is premised that the negative pressure applied to the sheet from the suction device be maintained constant and invariable. Hence, if the thickness of the paper or the paper quality of the sheets changes, the attraction and separation of a sheet may not be successfully achieved due to the failure of applying a negative pressure of an appropriate magnitude to a particular sheet. If the negative pressure applied is adjustable, the degree of adjustment which can be achieved is on the order of that allowed by manual operation of a valve, which is insufficient to satisfy the requirements for a sheet separation apparatus which may be used in a printing machine or a copying machine where sheets having different thickness or paper quality have to be separated and conveyed, leaving a problem to be overcome.
The problem can be appreciated by considering the situation when a relatively low negative pressure is established. In this instance, a failure to feed a sheet having a high air permeability is likely to occur. Conversely, when a relatively high negative pressure is established, more than one sheet may be fed in superimposed relationship. If a manual adjustment of a valve is relied on to adjust the amount of negative pressure, it is necessary to determine a magnitude of the negative pressure which is suitable for particular sheets used through previous experiments. In addition, the adjustment is a time consuming and troublesome operation inasmuch as the valve must be adjusted each time the thickness or quality of the paper changes.